Migration of the proton in the strong O-H.O hydrogen bond in urea-phosphoric acid (1/1).

The structure of urea-phosphoric acid is reported at a large number of temperatures in the range 150-335 K from neutron diffraction data collected using a novel multiple single-crystal data collection method. The work focuses on the behaviour of the H atom involved in the short strong O-H.O hydrogen bond in this material. The position of this atom is shown to vary significantly, by around 0.035 A, as a function of temperature, becoming effectively centred at the highest temperatures studied. This result, only accessible due to the accurate determination of H-atom parameters by neutron diffraction, has implications for the potential governing the hydrogen bond.